1. Field of the Invention
The present invention relates to a method of detecting states of a storage device, and more particularly to a storage device state detection method of detecting the state of health and state of charge of a storage device.
2. Description of the Related Art
Storage devices such as lithium-ion secondary batteries and electric double-layer capacitors are used in various applications; for example, they are widely used in battery packs in mobile telephones, batteries in personal computers (PCs), and batteries in automobiles. In this case, it is very important to detect states of a storage device such as, for example, the state of health (SOH) and the state of charge (SOC). Particularly as for automobiles, detection of states of a storage device in an energy-saving automobile, which performs idle reduction, a hybrid automobile, an electric car is closely related to the running of the automobile, attracting attention as being very important.
A generally well-known method of detecting states of a storage device is to measure the voltage, current, and temperature of the storage device and calculate its SOH and SOC. Of these measurement parameters, the temperature of the storage device is very important because the temperature of the storage device largely affects storage device state detection.
As related art of a method of measuring the temperature of a storage device, a method of directly measuring the temperature with a temperature detecting element attached or connected to the storage device, as described in Japanese Unexamined Patent Application Publication No. 6-260215, is generally known. In Japanese Unexamined Patent Application Publication No. 6-260215, a zener diode is used as the temperature detecting element; the zener diode is connected to the positive terminal of a storage device so that a temperature transferred from the positive terminal of the storage device can be accurately measured. In Japanese Unexamined Patent Application Publication No. 6-260215 (first conventional example), however, there has been the problem that a large difference may occur between a temperature in the storage device and a temperature at a temperature detection point (in the first conventional example, the positive terminal of the storage device) of the temperature detecting element, in which case an accurate temperature of the storage device cannot be measured.
By contrast, in a device proposed in Japanese Unexamined Patent Application Publication No. 2010-67502, the temperature of a storage device is detected by using a temperature detecting element such as a thermocouple, the internal impedance of the storage device is obtained, and the SOH of the storage device is decided. In a device proposed in U.S. Pat. No. 6,137,269, the internal impedance of the storage device is obtained without using a temperature detecting element, and the internal temperature of the storage device is determined.
A storage device in Japanese Unexamined Patent Application Publication No. 2010-67502 (second conventional example) includes a temperature adjusting means for adjusting the temperature of the storage device, an impedance calculating means for calculating the internal impedance of the storage device, and a deterioration deciding means for deciding the SOH of the storage device according to the calculated internal impedance. The SOH can be decided with reference to an internal impedance map that has been preset in correspondence to the temperature of the storage device and its voltage according to the voltage of the storage device and its temperature and to an internal impedance value calculated in a state in which the storage device has been adjusted to a temperature within a prescribed range by the temperature adjusting means when the storage device is charged or discharged with an alternating current at a prescribed frequency (at least 10 Hz and at most 1 kHz).
The device, in U.S. Pat. No. 6,137,269 (third conventional example), that determine an internal temperature includes an electric excitation circuit that applies time-varying electric excitation to a storage device, a response detecting circuit that detects an electric response that varies with time as a result of the time-varying electric excitation, and a calculation circuit that determines an internal temperature by using a voltage and current signal derived from entered excitation and an entered response signal. Time-varying electric excitation is provided at different frequencies (in an embodiment, 5 Hz, 70 Hz, and 1 kHz), a measured impedance is assigned to an assumed equivalent circuit, and the internal temperature of the storage device is calculated from a particular element value of the equivalent circuit.